The need for low emissions fuels has created an increased demand for light olefins for use in alkylation, oligomerization, MTBE and ETBE synthesis processes. In addition, a low cost supply of light olefins, particularly propylene, continues to be in demand to serve as feedstock for polyolefin, particularly polypropylene production.
Conventional fluidized and fixed bed processes may be used for increasing olefin production. For example, U.S. Pat. No. 4,830,728 discloses an FCC unit that is operated to maximize olefin production. The FCC unit has two separate risers into which a different feed stream is introduced. The operation of the risers is designed so that a suitable catalyst will act to convert a heavy gas oil in one riser and another suitable catalyst will act to crack a lighter olefin/naphtha feed in the other riser. Conditions within the heavy gas oil riser can be modified to maximize either gasoline or olefin production. The primary means of maximizing production of the desired product is by using a specified catalyst.
Conventional hydrocarbon conversion processes may utilize crystalline zeolites as catalysts. Crystalline zeolites have been found to be effective for a wide variety of hydrocarbon conversion processes including the catalytic conversion of naphthas to produce light (C.sub.2 to C.sub.4) olefins. Conventionally, crystalline zeolite is incorporated into a matrix in order to form a catalytic cracking catalyst.
Also, U.S. Pat. No. 5,026,936 to Arco teaches a process for the preparation of propylene from C.sub.4 or higher feeds by a combination of cracking and metathesis wherein the higher hydrocarbon is cracked to form ethylene and propylene and at least a portion of the ethylene is metathesized to propylene. See also, U.S. Pat. Nos. 5,026,935; 5,171,921 and 5,043,522.
U.S. Pat. No. 5,069,776 teaches a process for the conversion of a hydrocarbonaceous feedstock by contacting the feedstock with a moving bed of a zeolitic catalyst comprising a zeolite with a pore diameter of 0.3 to 0.7 nm, at a temperature above about 500.degree. C. and at a residence time less than about 10 seconds. Olefins are produced with relatively little saturated gaseous hydrocarbons being formed. Also, U.S. Pat. No. 3,928,172 to Mobil teaches a process for converting hydrocarbonaceous feedstocks wherein olefins are produced by reacting said feedstock in the presence of a ZSM-5 catalyst.
There remains a need, though, for more active and selective catalysts useful for catalytically converting naphtha into light olefins, especially propylene.
Conventional silicoaluminophoshates such as SAPO-11 are catalytically active molecular sieves, especially at high silicon concentrations. However, such materials are prone to detrimental silicon distribution in the framework at high silicon concentration, above about 0.04 molar Si fraction in the framework, resulting in a diminished catalytic activity. Moreover, even materials with a favorable Si distribution in the framework may undergo an undesirable redistribution of framework Si during, for example, catalyst regeneration. There is therefore a need for improved silicoaluminophoshates that have increased catalytic activity at high silicon concentration, and improved activity maintenance characteristics.